Motor vehicle brake system comprising a parking brake function and electromechanical wheel brake for such a motor vehicle brake system

ABSTRACT

The present invention relates to a motor vehicle brake system, having at least one first and at least one second electromechanical wheel brake ( 10 ), which each comprise an electric actuator for generating an actuating force and a self-boosting device for automatically boosting the actuating force generated by the actuator in order to press a friction element against a rotatable component ( 14 ) of the wheel brake ( 10 ) that is to be braked, wherein each self-boosting device comprises a wedge ( 18 ), which is supported against an associated abutment ( 22 ) and has at least one wedge face ( 20, 20 ′) disposed at an angle of slope (α). In order to realize a parking brake function, the self-boosting device of the first wheel brake comprises at least one wedge face ( 20 ), which is used to boost the force in brake operations during forward travel, and the self-boosting device of the second wheel brake comprises at least one wedge face ( 20 ′), which is used to boost the force in braking operations during reverse travel. In many embodiments, the angle of slope (α) of the said wedge faces ( 20, 20 ′) is so selected that the wheel brakes ( 10 ) in any case with normally prevailing coefficients of friction μ are self-locking. In the parking brake function the friction element of the first wheel brake, by utilizing the wedge face ( 20 ) used to boost the force in braking operations during forward travel, and the friction element of the second wheel brake, by utilizing the wedge face ( 20 ′) used to boost the force in braking operations during reverse travel, are clamped against the component of the wheel brake ( 10 ) that is to be braked. The invention also relates to an electromechanical brake for such a motor vehicle brake system.

The invention relates to a motor vehicle brake system with self-boostingelectromechanical wheel brakes and to a self-boosting electromechanicalwheel brake for such a motor vehicle wheel brake system.

Self-boosting electromechanical wheel brakes are known as such, e.g.from the German patent specification DE 198 19 564 C2. Anelectromechanical wheel brake with self-boosting comprises an electricactuator, which generates an actuating force and transmits it via aself-boosting device to a friction element in order to press thefriction element against a rotatable component of the wheel brake thatis to be braked. The self-boosting device comprises a wedge-shapedelement having a wedge face disposed at an angle of slope α. Acorresponding friction-element-side wedge face formed e.g. on thefriction lining carrier interacts with the wedge face of the wedgeelement in that the rotating component of the wheel brake that is to bebraked drives the friction element, which during the braking operationis pressed against the component to be braked, slightly in the directionof rotation, with the result that the two wedge faces move relative toone another and the friction element is pressed even more stronglyagainst the wheel brake component to be braked, without an increase ofthe actuating force supplied by the actuator being required for thispurpose. The angle of slope α of the wedge faces may in said case be soselected that during normal braking operations an actuating force needbe summoned up only initially by the actuator in order to press thefriction element against the brake component to be braked and that at afurther stage of the braking operation no actuating forces or at leastonly slight actuating forces are required.

The characteristic feature of self-boosting electro-mechanical brakes,namely the self-boosting device, makes it impossible to borrow conceptsfor achieving a parking brake function that are known from directlyactuated electromechanical wheel brakes.

The underlying object of the invention is, in a motor vehicle brakesystem using self-boosting electromechanical wheel brakes, to provide aparking brake function and to indicate a self-boosting electromechanicalwheel brake that is suitable for such a motor vehicle brake system.

According to the invention this object is achieved by a motor vehiclebrake system having the features indicated in claim 1. Accordingly, inorder to realize a parking brake function the self-boosting device of atleast one first wheel brake has at least one wedge face, which isdisposed at an angle of slope and used for boosting the force in brakingoperations during forward travel, while the self-boosting device of atleast one second wheel brake has at least one wedge face, which isdisposed at an angle of slope and used for boosting the force in brakingoperations during reverse travel. In the parking brake function thefriction elements of the first wheel brake are clamped against thecomponent of the wheel brake to be braked by utilizing the wedge faceused to boost the force in braking operations during forward travel, andthe friction elements of the second wheel brake are clamped against thecomponent of the wheel brake to be braked by utilizing the wedge faceused to boost the force in braking operations during reverse travel. Itis thereby ensured that a vehicle secured by the parking brake functionof the motor vehicle brake system according to the invention isincapable of movement either in forward direction or in backwarddirection. A force pushing the vehicle in forward direction would namelylead to an automatic stronger application of the first wheel brakewhile, conversely, a force pushing the vehicle in backward directionwould lead to a stronger application of the second wheel brake.

As a rule, particularly in two-axled vehicles, the first wheel brakewill be a front wheel brake and the second wheel brake will be a rearwheel brake. However, the first and the second wheel brake mightalternatively be disposed on one and the same axle of a vehicle, e.g. ona live axle of a tractor.

In principle, to achieve the parking brake function it is alreadysufficient when in each case a first and a second wheel brake, which arepreferably disposed diagonally opposite on the vehicle, are applied inthe described manner. Normally, however, to increase the brake forceachieved in the parking brake position two first wheel brakes and twosecond wheel brakes, preferably two front and two rear wheel brakes,will be applied in the described manner. Alternatively, it is alsopossible in the parking brake position to apply the wheel brakesaxle-wise in such a way that in the parking brake function the one wheelbrake utilizes the wedge face used to boost the force during forwardtravel and the other brake wheel utilizes the wedge face used to boostthe force during reverse travel. This however presupposes that all ofthe wheel brakes are equipped in each case with wedge faces both forboosting the force in braking operations during forward travel and forboosting the force in braking operations during reverse travel. If themotor vehicle has a plurality of rear axles, the wheel brakes of all ofthe rear axles or of only some of the rear axles may be used to realizethe parking brake function.

It should be expressly pointed out at this point that the motor vehiclebrake system according to the invention is intended primarily for theimplementation of service braking operations, and that the parking brakefunction realized in accordance with the invention is an additionalfunction of the brake system. Since the front wheel brakes during normalservice braking operations have to summon up up to 80% of the brakeforce, in simple embodiments of the motor vehicle brake system accordingto the invention the rear wheel brakes may be designed in such a waythat they do not participate in forward braking operations, i.e. eachrear wheel brake has only one wedge face, which is used to boost theforce in braking operations during reverse travel. Since during reversetravel the maximum speed of a vehicle is limited, in such a simpleembodiment the front wheel brakes may moreover be designed in such a waythat they do not participate in a braking operation of a backward movingvehicle, i.e. the front wheel brakes have only such wedge faces as areused to boost the force in braking operations during forward travel. Ifa motor vehicle brake system is required to meet higher standards, e.g.if it is fitted in high-speed and/or heavy vehicles, then usually atleast the rear self-boosting electromechanical wheel brakes are designedin such a way that they may brake in both directions of travel, i.e.these wheel brakes have wedge faces for forward travel and reversetravel. Because the front wheel brakes, owing to the dynamic axle loaddisplacement that occurs during normal operation (forward travel), arein any case designed to cope with higher brake forces, the brake forcethat may be generated by such a brake counter to the wedge direction,i.e. during reverse travel, is also mostly adequate, especially as thedynamic axle load displacement that occurs in a braking operation duringreverse travel removes load from the front axle and prevents thebuild-up of higher brake forces.

According to a preferred refinement of the motor vehicle brake systemaccording to the invention, in which the angle of slope of the saidwedge faces is selected in such a way that the wheel brakes areself-locking in any case given normally prevailing coefficients offriction between the friction element and the wheel brake component tobe braked, in order to achieve the parking brake function initially adistance-controlled application of each wheel brake participating in theparking brake function is effected. Here, what is meant by the term“distance-controlled application” is that, in order to achieve theparking brake function, each wheel brake initially travels apredetermined infeed distance in order thereby to achieve a desireddefined amount of holding force. The predetermined infeed distance maynot be too large in order to avoid too strong an application of thewheel brake, which might occur e.g. when a brake is very hot owing tomany preceding service braking operations. The distance-controlledapplication of each wheel brake is followed by a further actuation ofthe actuator of the brake with only low force, described as “zero-force”correction, in order subsequent to the application of the brake torestore a specific actuating clearance relative to the wedge of theself-boosting device that enables the wedge, when external forces actupon the vehicle and endeavour to shift the vehicle, to apply the wheelbrake even more strongly. Here, by the term “zero-force” is meant aforce that, compared to the previously occurring brake application, isnegligibly low.

According to a preferred embodiment of the previously described motorvehicle brake system, the distance-controlled brake application iseffected over a predetermined infeed distance up to a predeterminedbrake application force and is terminated when either the predeterminedinfeed distance or the predetermined brake application force has beenreached. This reliably prevents too strong an application of a wheelbrake that is very hot owing to the preceding operation. Because, when awheel brake is hot, the components participating in the brakingoperation expand, an exclusively distance-controlled brake applicationwould lead to a brake application force that was clearly too high. Theadditional monitoring of the brake application force achieved during thebrake application operation prevents this by terminating the brakeapplication operation when the predetermined brake application force hasbeen reached, even if the predetermined infeed distance has not yet beentravelled. In such an embodiment, the zero-force correction of theactuator of the brake occurs only when the predetermined infeed distancehas been reached. If the brake application operation has been terminatedowing to attainment of the predetermined brake application force, it isnamely impossible to restore an actuating clearance at the wedge by a“zero-force” correction of the actuator.

The previously mentioned, predetermined brake application force isadvantageously a fraction of the maximum actuator force, being forexample 30% of the maximum force that may be generated by the actuator.

In order to maintain the parking brake function, after a predeterminedtime interval and/or in dependence upon the temperature of brakecomponents relevant to the brake application a further application ofthe wheel brake is advantageously effected up to a predetermined brakeapplication force, which may be the same as the previously mentionedpredetermined brake application force. The predetermined time intervalis, for example, so selected that an initially hot brake may cool downand the force-controlled brake application then has, as it were, areclamping effect. Alternatively or additionally, the force-controlledbrake application may be effected in a temperature-dependent manner,e.g. in dependence upon the temperature of a brake disc, a brake caliperor similar brake components relevant to the application of the brake. Asan initially set brake application force decreases as the temperature ofthe brake falls, it is thereby guaranteed that a desired holding forceis reliably maintained even over extended periods of time. The furtherapplication of the wheel brake may comprise one or even a plurality ofbrake application operations.

It has already been mentioned that, in a self-boosting electromechanicalwheel brake, external forces acting upon the vehicle in the parkingbrake position may bring about an automatic further application of thewheel brake. This is possible because the wedge arrangement, which isused for self-boosting and upon which the actuator of the brake acts,has a specific clearance between the actuating element of the actuatorand the wedge, upon which the actuating element of the actuator acts. Anautomatic further application of the wheel brake in parking brakeposition may therefore occur until the said clearance is used up. Inorder, after external forces have acted upon a vehicle secured by meansof the parking brake function of the motor vehicle brake systemaccording to the invention, to restore a state, in which a furtherautomatic application of the wheel brakes owing to external forces ispossible, according to a preferred refinement a zero-force correction ofthe actuator of each wheel brake participating in the parking brakefunction is effected when the clearance existing in immobilizingdirection between the actuator and the wedge has been used up. Thezero-force correction is therefore intended only to restore the saidclearance, not however summon up an additional immobilizing force.

In the embodiments described above, a reclamping of the brake isoptionally provided in order to compensate the clamping force reductionthat occurs upon cooling of a hot brake and to guarantee that thevehicle is reliably held in place by means of the brake(s) situated inparking brake position. Under quite particularly adverse operatingconditions, in particular given an extremely hot brake, the coefficientof friction μ may be lower than the tangent of the wedge angle α, withthe result that the brake is no longer self-locking. Under suchcircumstances, a braking action in the parking brake position isachievable only by means of a continuous actuator force. This isunsatisfactory owing to a continuous requirement for electrical energy.

For this reason, according to a modified embodiment of the motor vehiclebrake system according to the invention, spring-elastic means aredisposed between the actuator, which applies the brake in the parkingbrake function, and the associated friction element and the actuator isof an arrestable, i.e. lockable design. The spring-elastic means may beformed e.g. by a helical spring. Preferably, the spring-elastic meansact upon the wedge arrangement of the self-boosting device, i.e. thespring-elastic means are supported against the then locked actuator andensure a continuous actuating force, without energy being continuouslyrequired to generate this actuating force. The desired immobilizingeffect of the parking brake is therefore guaranteed also under extremelyunfavourable operating conditions without a continuous supply ofelectrical energy. Given such a refinement, the angle of slope of thedescribed wedge faces need not be selected in such a way that the wheelbrakes are self-locking in any case with normally prevailingcoefficients of friction between the friction element and the wheelbrake component to be braked.

According to a preferred development of the motor vehicle brake systemaccording to the invention, on gradients an activation of the parkingbrake function is effected automatically after attainment of thestationary state of the vehicle from forward travel. In this way, anunintentional backward rolling of the vehicle is substantiallyprevented. To enable the motor vehicle brake system to detect whetherthe vehicle is travelling uphill, either a suitable sensor, e.g. aninclination sensor, may be provided or the information of anothervehicle system, which comprises a sensor capable of detectinginclinations of the vehicle, may be used. For example, modern anti-theftalarm systems are often equipped with an inclination sensor.

In an advantageous refinement of the embodiment just mentioned,activation of the parking brake function is effected only after theservice brake has been released. The thinking behind this is thatbackward rolling of the vehicle is unable to occur so long as theservice brake is actuated, so that this refinement prevents unnecessaryactivation of the parking brake function. According to a furtherdeveloped refinement, activation of the parking brake function is noteffected until a predetermined length of time after release of theservice brake. This is to enable the driver of a vehicle, after thestationary state of the vehicle has been reached, to execute a normalstart-up operation, e.g. by engaging first gear and then releasing theclutch, without the parking brake function having already beenactivated. It is only after a predetermined length of time, which mayfor example be so selected that a normal start-up operation is bound tohave been completed, that activation of the parking brake function isoptionally effected, i.e. if the vehicle rolls backwards. Unintentionalbackward rolling of the vehicle for a longer period is thereforeprevented.

In addition to triggering the parking brake function after attainment ofthe stationary state of the vehicle from forward travel, in modifiedembodiments of the previously described motor vehicle brake system anactivation of the parking brake function may also be effectedautomatically after attainment of the stationary state of the vehiclefrom reverse travel. So that intentional reverse travel is possible,this automatic activation of the parking brake function is effectedpreferably only when no gear of the vehicle transmission is engaged.Here, by “gear” is meant in particular reverse gear and/or first gear.

According to a further modification of the previously discussed motorvehicle brake system, in which activation of the parking brake functionon gradients is effected automatically at least after attainment of thestationary state of the vehicle from forward travel, this activation ismoreover effected only when a predetermined friction torque in backwardrolling direction is exceeded. An automatic activation of the parkingbrake function accordingly occurs only when, despite the service brakebeing actuated, a backward rolling of the vehicle occurs. It is thenassumed that such backward rolling is an unintended event, and theparking brake function that then comes into effect prevents furtherbackward rolling. The friction torque in backward rolling direction maybe determined e.g. with the aid of sensors that are in any case providedat the self-boosting electromechanical wheel brake for friction torquecontrol during a service braking operation.

All embodiments of the motor vehicle brake system according to theinvention are preferably refined in such a way that an activation of theparking brake function is effected automatically with switching-off ofthe motor vehicle engine. The thinking behind this is that, after themotor vehicle engine is switched off, no further movement of the vehicleis desired. For special situations, e.g. to enable pushing or towing ofa broken-down vehicle, there is then preferably a switch, which effectsthe overriding of the automatic activation.

The present invention also provides an electromechanical wheel brake foruse in a motor vehicle brake system according to the invention,comprising an electric actuator for generating an actuating force and aself-boosting device for automatically boosting the actuating forcegenerated by the actuator in order to press a friction element against arotatable brake component that is to be braked, e.g. against a brakedisc. The self-boosting device comprises a wedge, which is supportedagainst an associated abutment and has at least one wedge face disposedat an angle of slope that is so selected that the wheel brake in anycase with normally prevailing coefficients of friction between frictionelement and brake disc is self-locking. The actuator has two drives,which are designed in such a way that they may act in the same directionor in opposite directions upon the wedge in order during service brakingoperations to enable a backlash-free actuation of the wedge, and inorder moreover in a parking brake position of the brake to be able togive the wedge a clearance that enables an automatic further applicationof the wheel brake by means of external forces acting upon the vehicle.

The friction element or each friction element is preferably providedwith a friction lining, which has a large jump in the coefficient ofadhesion between static and sliding friction. In conventional brakes alarge jump in the coefficient of adhesion between static and slidingfriction is not desired, rather this jump in the coefficient of adhesionis to be as small as possible there in order to achieve as smooth aspossible a braking operation up to the stationary state of the vehicle.In a wheel brake according to the invention, on the other hand, a largejump in the coefficient of adhesion is advantageous especially for theparking brake function because, after the first distance-controlledapplication of each wheel brake in the parking function, this large jumpprevents the action of even small external forces upon the stationaryvehicle from causing a “slipping” of the wheel brakes and hence amovement of the vehicle. What is more, the fact that the static frictionis clearly greater than the sliding friction ensures that in the parkingbrake function the automatic further application of the wheel brakesowing to external forces acting upon the vehicle occurs.

As already mentioned, under very adverse operating conditions, inparticular when a brake is extremely hot, the coefficient of staticfriction μ between the friction element and the brake disc may becomelower than the tangent of the wedge angle α, with the result that thebrake is then no longer self-locking. This means that in the parkingbrake position an actuator force has to be summoned up continuously inorder to achieve the desired immobilizing effect. To avoid this,according to an embodiment of the electromechanical wheel brakeaccording to the invention it is provided that the actuator of the brakeis selectively arrestable, i.e. lockable, and that between the actuatorand the friction element spring-elastic means act and exert a brakeapplication force upon the friction element. The spring-elastic meansmay be formed e.g. by a helical spring that exerts pressure in theparking brake position. In the parking brake position the spring-elasticmeans are supported against the actuator, which is then locked againstdisplacement, and continuously exert a force upon the friction element.Preferably, the spring-elastic means act upon the wedge of theself-boosting device of the electromechanical brake according to theinvention.

According to an embodiment, the electric actuator of the wheel brakecomprises two drives in the form of linear actuators, which are bothworkingly connected in each case by a push rod to the wedge of theself-boosting device. The two linear actuators upon a brake actuationmay operate in the same direction or in opposite directions, dependingon the operating state. In one embodiment, the spring-elastic means,which exert a pressing force in the parking brake position, are disposedbetween the ends of the push rods of the two linear actuators. Inanother embodiment, the spring-elastic means are disposed between theend of the push rod of the linear actuator that brings the brake intothe parking brake position and the wedge arrangement of theself-boosting device. In all of the previously described embodiments,the spring-elastic means ensure the desired immobilizing effect of thewheel brake also under the discussed adverse operating conditions andwithout a power supply.

In a preferred embodiment of the self-boosting electro-mechanical wheelbrake according to the invention, for releasing the wheel brake from theparking brake position under adverse operating conditions, e.g. afterlong stationary periods, there is an additional separate drive. Thisdrive preferably comprises a step-down worm gear, so that the drive maybe kept small. In an advantageous refinement, the worm gear and theseparate drive are components of an adjusting device of the wheel brakefor compensating friction lining wear.

Embodiments of a brake according to the invention are described indetail below with reference to the accompanying diagrammatic drawings.The drawings show:

FIG. 1 a side view of an electromechanical brake according to theinvention in the form of a disc brake,

FIG. 2 a three-dimensional view of the brake according to the inventionobliquely from below,

FIG. 3 the view of FIG. 2 without adjusting apparatus and abutment,

FIG. 4 the section II—II from FIG. 1,

FIG. 5 the section III—III from FIG. 1,

FIG. 6 the section IV—IV from FIG. 4,

FIG. 7 a sectional view according to FIG. 4 showing the actuated stateof the brake according to the invention during forward travel,

FIG. 8 the sectional view of FIG. 7, now however showing the actuatedstate of the brake during reverse travel,

FIG. 9 the section V—V from FIG. 4,

FIG. 10 the sectional view of FIG. 4 with extensively worn frictionlinings,

FIG. 11 a diagram illustrating the basic function of the brake accordingto the invention,

FIG. 12 a sectional view of a modified embodiment of anelectromechanical brake according to the invention in the form of a discbrake, wherein in FIG. 12 the released state of the brake is shown,

FIG. 13 the view of FIG. 12 in an actuated state of the brake, and

FIG. 14 an embodiment modified compared to FIGS. 12 and 13.

FIGS. 1 and 2 show an electromechanical brake 10 in the form of a discbrake, comprising a housing 12 and a brake disc 14 rotatable about anaxis A.

As may be seen more clearly from FIGS. 3, 4 and 5, the brake 10 has afirst friction lining 16, which is firmly connected, e.g. glued, to thefront of a wedge 18 serving as a lining carrier. At its rear, the wedge18 has for each direction of rotation of the brake disc 14 a wedge face20 and/or 20′, which are both disposed at an angle of slope α relativeto the brake disc 14 and supported against complementary wedge faces 21,21′ of a block-shaped abutment 22.

The abutment 22 is supported by means of four threaded bolts 24 againsta brake caliper 26 (see FIGS. 2 and 5), which spans the brake disc 14and has an arm 28 directed towards the axis of rotation A. The arm 28 isused to support a second friction lining 30, which is fastened in aconventional manner on a brake lining plate 32, which lies adjacent tothe inner side of the arm 28 facing the brake disc 14.

The actuating force of the brake 10 is generated by an electricactuator, which comprises two drives 34 and 34′ designed here as linearactuators. Each drive 34, 34′ comprises an electric motor 36, 36′ and apush rod 38, 38′ driven thereby and workingly connected to the wedge 18.In the embodiment illustrated here, each electric motor 36, 36′ has anintegrated spindle nut (not shown) and the push rods 38, 38′ aredesigned in each case as a spindle interacting with the spindle nut. Bymeans of a likewise non-illustrated angle-of-rotation sensor in eachelectric motor 36, 36′ the exact position of the associated push rod 38,38′ may be determined on the basis of the revolutions executed by theelectric motor 36 or 36′ and the lead of the spindle s mechanism.

The wedge 18 and the abutment 22 are part of a self-boosting device forboosting the actuating force generated by the drives 34, 34′. For thispurpose, the free ends of the push rods 38 and 38′ are mounted in such away in a receiver 40 disposed at the rear of the wedge 18 that atranslational movement of the push rods 38, 38′ leads to a correspondingdisplacement of the wedge 18 to the left or to the right (see FIGS. 3, 4and 6). To actuate the brake 10, therefore, the wedge 18 with thefriction lining 16 fastened thereon is displaced in the direction ofrotation of the brake disc 14 (see FIGS. 7 and 8), namely by means of atranslational movement of the two push rods 38 and 38′. In said case,the wedge 18 is supported by its one wedge face 20 or 20′ against theassociated, complementary wedge face 21 or 21′ of the abutment 22 andmoves not only to the left or right but also towards the brake disc 14.As soon as the first friction lining 16 comes into contact with thebrake disc 14, a reaction force arises and is transmitted from thefriction lining 16 via the wedge 18 and the abutment 22 to the brakecaliper 26. The brake caliper 26 is float-mounted on the housing 12 ofthe brake 10 and is displaced by the said reaction force until thesecond friction lining 30 is likewise applied against the brake disc 14(floating caliper principle). Each further translatory displacement ofthe wedge 18 in actuating direction then leads to a stronger applicationof the two friction linings 16 and 30 against the brake disc 14 andhence to the desired braking operation. Release of the brake is effectedby displacement of the wedge 18 back into its original position shown inFIG. 4. To reduce the friction, the wedge faces 20, 20′ and/or theabutment faces 21, 21′ may be provided e.g. with rolling bodies (notshown). As illustrated, the receiver 40 is designed in such a way thatthe wedge 18 may move towards and away from the brake disc 14 withoutthe push rods 38, 38′ simultaneously executing this movement.

As FIG. 6 reveals, the heads 39 and 39′ forming the free ends of thepush rods 38 and 38′ lie in actuating direction against correspondingfaces of the receiver 40. The dimensions of the push rod heads 39, 39′are so selected that in the receiver 40 at the side of each head 39, 39′remote from the actuating direction there is a clearance s. After thebrake has been applied to achieve a parking brake function, thisclearance s makes it possible for external forces, which are acting uponthe vehicle and endeavouring to shift it, to effect an automatic furtherapplication of the brake, thereby preventing an unintentional movementof the vehicle. During an automatic further brake application effectedbecause of external forces acting upon the vehicle the clearance s isused up, i.e. an automatic further application of the brake may occuronly up to the point when the faces of the receiver 40, which wereoriginally not in contact with the heads 39 and 39′, strike against theheads 39, 39′. The clearance s has then relocated to the respectiveother side of the heads 39, 39′. By means of a practically zero-forcecorrection of the push rod heads 39 and 39′ the originally existingstate may be restored, i.e. the clearance s may be shifted back to theside of the heads 39, 39′ illustrated in FIG. 6. Once the originalclearance s has been restored in this way, there is no longer anythingto prevent a further automatic application of the brake.

The “zero-force” correction of the push rod heads 39, 39′ or, in moregeneral terms, of the electric actuator occurs after adistance-controlled brake application that is effected at the start of aparking brake function in order to apply the friction linings againstthe brake disc 14 and achieve a specific minimum brake applicationforce. Because the correction is effected with a negligibly low force,the push rod heads 39, 39′ are actually moved only when the originallyexisting clearance s has already been used up, e.g. because the vehicleis parked on a slope and the slope output force acting upon the vehiclehas led, immediately after the initially effected distance-controlledbrake application, to an automatic further application of the brake. If,on the other hand, the original clearance s still exists, the“zero-force” correction does not lead to any change of the conditionsand is terminated after a predetermined time interval has elapsed.

So that the brake 10 may compensate the wear of a friction lining 16, anadjusting device generally denoted by 42 is provided (see FIG. 2). Thisdevice comprises (see FIGS. 4, 5 and 9) a motor 44, which drives a wormshaft 46, which is in mesh with four gear wheels 48. The gear wheels 48are mounted in the brake caliper 26 and each have an internal thread,which is in engagement with an appropriate one of the threaded bolts 24firmly connected to the abutment 22 (see FIG. 5). The gear wheels 48accordingly operate as spindle nuts of a spindle mechanism, while thethreaded bolts 24 are the spindle rods. In the illustrated embodimentthere are four threaded bolts 24, of which because of the differentdirection of rotation of the gear wheels 48 two threaded bolts 24 have aleft-hand thread and the other two threaded bolts 24 have a right-handthread. By means of the motor 44 the adjusting device 42 may thereforeincrease the distance of the abutment 22 from the brake caliper 26, i.e.move the abutment 22 towards the brake disc 14. Thus, the releaseclearance of the brake 10, i.e. the distance that exists between thebrake disc 14 and the friction lining surface in the released state ofthe brake, may be kept constant. FIG. 10 shows this in a view accordingto FIG. 4, but with extensively worn friction linings 16, 30.

Usually the brake 10 will be so designed that, if during a brakingoperation too large a release clearance is detected, a closed-loopcontrol circuit activates the adjusting device 42 in the released stateof the brake in order to reduce the release clearance back to the designspecification value. The adjusting device 42 is preferably of aself-locking design in order to prevent an unintended adjustment of therelease clearance.

The adjusting device 42 described here is one possibility ofcompensating the friction lining wear. Other embodiments of the brake 10may have, instead of the said electric motor 44, an ultrasonic motor, asequence processor, a stepping motor or another drive. The gear of theadjusting device 42 may also be designed differently, e.g. in the formof a harmonic drive gear. Furthermore, there need not be four threadedbolts 24, as illustrated, rather there may be a higher or lower numberof threaded bolts and, finally, means other than threaded bolts areconceivable for achieving the described relative displacement of theabutment 22.

There now follows a detailed functional description of theelectromechanical brake 10 and, in particular, of the self-boostingdevice with reference to FIG. 11. It has already been mentioned that theself-boosting device has, for each direction of rotation of the brakedisc 14, a wedge face 20 and/or 20′ that is supported against acomplementary face 21 and/or 21′ of the abutment 22. In the illustratedembodiment, each wedge face 20, 20′ is disposed at an effective wedgeangle α relative to the brake disc 14. However, this need notnecessarily be the case, rather the effective wedge angle for the onedirection of rotation may differ from the effective wedge angle for theother direction of rotation. In FIG. 11 arrows indicate the forces thatact upon the wedge 18.

These are

-   F_(A) the input force introduced into the wedge 18,-   F_(R) the supporting force that arises during a braking operation    and is to be supported by the abutment 22 and may be divided into a    force F_(Rx) opposed to the input force F_(A) and a pressing force    F_(Ry) perpendicular to the brake disc,-   F_(N) the normal force acting on the brake disc in the opposite    direction to the force F_(Ry), and-   F_(F) the friction force arising at the wedge and/or at the friction    element.

According to this equilibrium of forces the friction force and/or thefriction torque at the brake disc 14 in accordance with the relationship

$F_{A} = {{- F_{F}} \cdot \lbrack {1 - \frac{\tan\mspace{14mu}\alpha}{\mu}} \rbrack}$depends only upon the angle of slope α, the coefficient of friction μrepresenting a disturbance variable, and the input force F_(A).

The input force F_(A), which acts according to FIG. 11 during a brakeactuation upon the wedge 18, is generated by the two drives 34, 34′.With a given coefficient of friction μ, the degree of self-boosting ofthe introduced force F_(A) depends only upon the angle of slope α: inthe state of equilibrium, i.e. when the value of the coefficient offriction μ equals the tangent of the angle of slope α, the brake 10—whenthe friction lining 16 is in contact with the brake disc 14—does notrequire any more input force F_(A) for the further braking operation.This state of equilibrium is therefore described as the optimumself-boosting point. If μ is lower than tan α, an input force F_(A) hasto be present to maintain a braking operation. If, on the other hand, μis greater than tan α, the brake runs in by itself, i.e. the brake forceis boosted without the presence of an input force F_(A) progressively upto locking of the brake. If this locking state is to be avoided and/or adesired brake force maintained, a negative input force F_(A), i.e. aninput force F_(A) acting in the opposite direction, has to be appliedonto the wedge 18.

So that the input force F_(A) may be low, it is desirable to operate thebrake 10 in a range, in which the coefficient of friction μ is at leastapproximately equal to the tangent of the angle of slope α. In thisrange of low actuating forces, the two drives 34 and 34′ operate counterto one another, i.e. the two drives 34, 34′ via the push rods 38, 38′introduce mutually opposed forces into the wedge 18. The opposed forcesare in said case so dimensioned that an excess of force results in thedirection, in which the wedge 18 is to be displaced upon an actuation.The two forces introduced by the drives 34, 34′ into the wedge 18 mayboth be pressing forces or both be tensile forces, all that matters isthat an excess of force results in the desired direction.

By virtue of the two drives 34, 34′ operating in opposite directions,the actuation of the wedge 18 is free from backlash. This freedom frombacklash is important for operation of the brake 10 in the optimumself-boosting range because in this range the variation of thecoefficient of friction μ during operation of the brake may lead to arapid change between states, in which μ is lower than tan α, and states,in which μ is greater than tan α. In other words, in the range aroundthe optimum self-boosting point there may be a rapid change betweenstates, in which a positive input force F_(A) is required, and states,in which a negative input force F_(A) is needed to maintain a specific,desired brake force. If the actuator were not free from backlash, ateach change of sign of the input force F_(A) the clearance existing inthe actuator would be travelled, which would lead to undefined statesand hence to poor controllability of the brake. The backlash-freeactuation by means of the two drives 34, 34′ operating normally inopposite directions effectively avoids this problem.

In operating states, in which the value of the coefficient of friction μdiffers greatly from the tangent of the angle of slope α, larger inputforces F_(A) are needed to achieve a desired braking effect. In suchoperating states, the two drives 34, 34′ operate with one another, i.e.they generate forces in the same direction in that one of the drivespresses upon the wedge 18 and the other drive pulls on the wedge 18. Toenable such an acting of the drives in the same direction, both drives34, 34′ are of a reversible design, i.e. their actuating direction maybe reversed. When the drives 34, 34′ are operating in the samedirection, the actuator of the brake 10 no longer operates free frombacklash. In practice, however, this is negligible since operatingstates, in which increased input forces F_(A) are needed, occur onlyrarely and, moreover, in such operating states a possible overtravel ofthe actuator clearance is tolerable.

As already briefly indicated, the coefficient of friction μ may varyrelatively strongly as a function of the load of the brake. Eachvariation of the coefficient of friction during a braking operationhowever leads to a variation of the friction force F_(F) and hence to avarying deceleration of the brake component to be braked, which isformed mainly by the brake disc 14. To correct these undesirablevariations of the coefficient of friction, the illustrated disc brake 10is provided with a non-illustrated sensor device, which allowscontinuous measurement of the friction force. This, as such, knownsensor device is connected to a likewise non-illustrated, electroniccontrol unit, which evaluates the received signals and in particularcarries out a comparison between a preset setpoint value of the frictionforce and the actual value of the friction force. In accordance withthis evaluation of the signals, the drives 34, 34′ are controlled by thecontrol unit in such a way that, by displacing the wedge 18 in orcounter to the direction of rotation of the brake disc 14, an increaseor decrease of the actual value of the friction force is achieved inorder to bring the friction force actual value into conformity with thefriction force setpoint value.

In the illustrated embodiment, control of the friction force of thebrake 10 is achieved through position control of the wedge 18. In termsof control technology this is advantageous because between the wedgeposition and the coefficient of friction μ there is merely a linearrelationship, which may be controlled easily, quickly and reliably, e.g.by means of a cascade control system comprising an external control loopand an internal control loop. In the external control loop the (desired)brake torque is the controlled variable, while the wedge position is themanipulated variable. In the internal control loop the wedge position isthe controlled variable, while the manipulated variable is the motorcurrent or alternatively the motor voltage of the electric motors 36,36′ of the drives 34, 34′. Because of the normally backlash-freeactuation of the wedge 18, the position of the wedge 18 may bedetermined precisely by means of the described angle-of-rotation sensorscontained in the electric motors 36, 36′.

In the illustrated embodiment, the angle of slope α is constant over theinfeed distance of the brake 10, more precisely of the wedge 18. Innon-illustrated embodiments, the angle of slope α is degressive, i.e.decreases with a progressive infeed distance.

FIG. 12 shows an embodiment of the brake 10 that is slightly modifiedcompared to the previously described embodiment. A spring 50 is disposedas a spring-elastic means between the push rod heads 39 and 39′. Upon anactuation of the brake 10 by means of the drives 34 and 34′ in the formof linear actuators, the spring 50 is compressed (see FIG. 13), so thatthe spring 50 is then able to exert a pressing force.

When, as illustrated in FIG. 13, for infeed of the brake 10, i.e. formoving the friction lining 16 towards the brake disc 14, the wedge 18 isto be displaced to the right, this occurs in that both drives 34 and 34′move the wedge 18 to the right. In the view reproduced in FIG. 13, thetensioned spring 50 presses upon the right drive 34′ and, via thelatter, upon the wedge 18. In order to achieve this, the left drive 34has to be arrested in the parking brake position, i.e. locked against atranslatory displacement, and the right drive 34′ may not be of aself-locking design. The arresting of the left drive 34 may be achievede.g. by means of a locking apparatus (not shown here) for the motor 36of the drive 34 or by means of a self-locking design of the spindlemechanism that interacts with the electric motor 36. The tensionedspring 50 accordingly in the parking brake position presses continuouslyupon the wedge 18 and thereby ensures the desired immobilizing effect ofthe brake 10 even if the coefficient of static friction μ because ofadverse operating conditions should have become so low that the brake 10is no longer self-locking.

As a rule, it is sufficient to design the brake 10 in such a way thatthe spring 50 may act only in relation to one direction of rotation ofthe brake disc 14, i.e. each brake 10 acts as a parking brake in onedirection only, e.g. the front wheels are secured against forward traveland the rear wheels against reverse travel. However, it is easily alsopossible to allow the spring 50 in the parking brake position to act inrelation to both directions of rotation of the brake disc 14.

FIG. 14 shows an embodiment of the brake 10, which is once more slightlymodified and in which the spring 50 is disposed, not between the twopush rod heads 39 and 39′, but between the push rod head 39 and a partconnected directly to the wedge 18. In such an embodiment, the rightdrive 34′ may also be of a self-locking design.

1. A motor vehicle brake system, comprising: at least one firstelectromechanical wheel brake; and at least one second electromechanicalwheel brake (10), each first and second electromechanical wheel brakecomprising an electric actuator for generating an actuating force, and aself-boosting device for automatically boosting the actuating forcegenerated by the actuator to thereby press a friction element against arotatable component (14) of the wheel brake (10), wherein eachself-boosting device comprises a wedge (18), which is supported againstan associated abutment (22) and has at least one wedge face (20, 20′)disposed at an angle of slope (α), the self-boosting device of the firstwheel brake comprising at least one wedge face (20), which is used toboost the force in braking operations during forward travel, theself-boosting device of the second wheel brake comprising at least onewedge face (20′), which is used to boost the force in braking operationsduring reverse travel, and wherein the friction element of the firstwheel brake, by utilizing the wedge face (20) used to boost the force inbraking operations during forward travel, and the friction element ofthe second wheel brake, by utilizing the wedge face (20′) used to boostthe force in braking operations during reverse travel, are clampedagainst the rotatable component of the wheel brake (10) to function as aparking brake.
 2. A motor vehicle brake system according to claim 1,wherein the angle of slope (α) of the said wedge faces (20, 20′) isselected in such a way that the wheel brakes (10) in any case withnormally prevailing coefficients of friction μ are self-locking.
 3. Amotor Meter vehicle brake system according to claim 2, wherein adistance-controlled application of each wheel brake (10) participatingin the parking brake function initially occurs and is followed by azero-force correction of the actuator of each wheel brake (10)participating in the parking brake function.
 4. A motor vehicle brakesystem according to claim 3 wherein the distance-controlled brakeapplication is effected over a predetermined infeed distance up to apredetermined brake application force and is terminated when either thepredetermined infeed distance or the predetermined brake applicationforce has been reached, and that the zero-force correction of theactuator is effected only when the predetermined infeed distance hasbeen reached.
 5. A motor vehicle brake system according to claim 4,wherein the predetermined brake application force is a fraction of amaximum actuator force, preferably 30% of the maximum actuator force. 6.A motor vehicle brake system according to claim 3, wherein apredetermined brake application force is effected after a predeterminedtime interval and/or in dependence upon a temperature of brakecomponents relevant to the brake application.
 7. A motor vehicle brakesystem according to claim 3, wherein a further zero-force correction ofthe actuator of each wheel brake (10) participating in the parking brakefunction is effected after the distance-controlled brake applicationexternal forces acting upon the vehicle have brought about an automaticfurther application of the wheel brake.
 8. A motor vehicle brake systemaccording to claim 1, wherein the actuator is lockable, the systemfurther comprising spring-elastic means between the actuator, and theassociated friction element of the wheel brake (10), wherein thespring-elastic means exert an application force upon the frictionelement in a parking brake position.
 9. A motor vehicle brake systemaccording to claim 8, wherein the spring-elastic means act upon a wedgearrangement of the self-boosting device that comprises the wedge face(20, 20′) and wherein the spring-elastic means are supported indirectlyor directly against the locked actuator.
 10. A motor vehicle brakesystem according to claim 8, wherein the spring-elastic means are formedby a helical spring.
 11. A motor vehicle brake system according to claim1, wherein an activation of the parking brake function is effectedautomatically on gradients after attainment of a stationary state of thevehicle from forward travel in order to combat a backward rolling of thevehicle.
 12. A motor vehicle brake system according to claim 11, whereinactivation of the parking brake function is effected only after aservice brake of the vehicle has been released.
 13. A motor vehiclebrake system according to claim 12, wherein activation of the parkingbrake function is not effected until a predetermined length of timeafter release of the service brake.
 14. A motor vehicle brake systemaccording to claim 11, wherein activation of the parking brake functionon gradients is effected automatically after attainment of thestationary state of the vehicle also from reverse travel.
 15. A motorvehicle brake system according to claim 14, wherein activation of theparking brake function is effected only when no gear of a transmissionof the vehicle is engaged.
 16. A motor vehicle brake system according toclaim 11, wherein activation of the parking brake function is effectedonly when a predetermined friction torque in the backward rollingdirection has been exceeded.
 17. A motor vehicle brake system accordingto claim 1, wherein activation of the parking brake function is effectedautomatically when an engine of the vehicle is switched off.
 18. Anelectromechanical wheel brake (10) for use in a motor vehicle brakesystem, comprising: an electric actuator for generating an actuatingforce and a self-boosting device for automatically boosting theactuating force, generated by the actuator in order to press a frictionelement against a rotatable component (14) of the wheel brake (10), theself-boosting device comprising a wedge (18), which is supported againstan associated abutment (22) and has at least one wedge face (20, 20′)disposed at an angle of slope (α), the actuator comprising two drives(34, 34′), which are so designed that they may act in the same directionor in opposite directions upon the wedge (18) to enable a backlash-freeactuation of the wedge (18) during service braking operation, and togive the wedge (18) a clearance in a parking brake position that enablesan automatic further application of the wheel brake (10) by means ofexternal forces acting upon the vehicle.
 19. A wheel brake according toclaim 18, wherein the angle of slope (α) is selected so that the wheelbrake (10) in any case with normally prevailing coefficients of frictionμ is self-locking.
 20. A wheel brake according to claim 18, wherein thefriction element comprises friction linings (16, 30) with a large jumpin the coefficient of adhesion between static friction and slidingfriction.
 21. A wheel brake according to claim 18, whereinspring-elastic means are disposed between the actuator, which appliesthe wheel brake (10) in the parking brake function, and the associatedfriction element of the wheel brake (10), and the spring-elastic meansexert an application force upon the friction element, and wherein theactuator is of a lockable design.
 22. A wheel brake according to claim21, wherein the actuator comprises an electric motor (36, 26′) with alocking apparatus.
 23. A wheel brake according to claim 21, wherein theactuator comprises a spindle mechanism of a self-locking design, whichinteracts with an electric motor (36, 36′).
 24. A wheel brake accordingto claim 21, wherein the spring-elastic means act upon a wedgearrangement of the self-boosting device that comprises the wedge face(20, 20′), and the spring-elastic means are supported indirectly ordirectly against the locked actuator.
 25. A wheel brake according toclaim 21, wherein the spring-elastic means are formed by a helicalspring, which exerts a pressure in the parking brake position.
 26. Awheel brake according to claim 21, wherein the drives (34, 34′) aredesigned as linear actuators which are workingly connected to the wedge(18) by a push rod (38, 38′) having a push rod head (39, 39′), andwherein the spring-elastic means, which in the parking brake positionexert an application force upon the friction element, are disposedbetween the push rod heads (39, 39′).
 27. A wheel brake according toclaim 21, wherein the drives (34, 34′) are designed as linear actuators,which are workingly connected to the wedge (18) by a push rod (38, 38′)having a push rod head (39, 39′), and wherein the spring-elastic means,which in the parking brake position exert an application force upon thefriction element, are disposed between the push rod head (39) of thelinear actuator that brings the wheel brake (20) into parking brakeposition and a wedge arrangement comprising the wedge (18).
 28. A wheelbrake according to claim 18, further comprising an additional separatedrive for releasing the wheel brake (20) from the parking brakeposition.
 29. A wheel brake according to claim 28, wherein the separatedrive includes a worm gear.
 30. A wheel brake according to claim 29,wherein the worm gear (46, 48) and the separate drive (44) arecomponents of an adjusting device (42) for compensating friction liningwear.